The disclosure concerns a method to operate an inkjet printer with at least one first and one second printing station to print to a continuous recording medium, as well as an inkjet printer with at least one first and one second printing station to print to a continuous recording medium.
High-capacity inkjet printers to print to a continuous recording medium normally have two printing stations that are arranged along a transport path of the recording medium. The recording medium is normally a paper web. A turning device is arranged between the two printing stations so that each printing station respectively prints to one of the two sides of the recording medium.
The printing stations have multiple print heads. In the movement direction of the recording medium, a drying device respectively follows the print heads in order to dry the recording medium printed with the liquid ink. The heating power of the drying device is set such that—during normal operation, during which the recording medium is moved with continuous speed—the recording medium is dried to such an extent that the moisture introduced with the ink is removed. If the operation of the inkjet printer is interrupted, so much heat is stored in the region of the drying device and deflection rollers following the drying device that a segment of the recording medium that is located in this region is more significantly dried out than is typical.
In an unprocessed state, paper has a specific basic moisture content. Given an interruption of the printing operation, a large part of this basic moisture content can be driven out of the paper. Since, during a longer pause of the operation of the inkjet printer, the recording medium is significantly dried only in the region of the drying device and in the region following the drying region in the movement direction, and retains its moisture in the remaining regions, transition regions arise in which a segment of normal moisture content and a significantly dried segment adjoin one another. The significantly dried segments are somewhat contracted relative to the segment of normal moisture, whereby there is warping in these transition regions. This warping forms waves.
That recording media can form waves in printing systems was already previously known. The inventor of the present Patent Application has conducted many series of tests and has more closely studied the cause of the wave formation. For the first time, he was able to more specifically define the location of the wave formation, namely adjacent to the transition region between segments of normal moisture content and significantly dried segments of the recording medium or paper. The phenomenon of wave formation was not previously known at this level of precision.
According to U.S. Pat. No. 6,837,635 B1, given a wave formation of the paper in an inkjet printer it is sought to keep the waves outside of the print region so that the print heads are not damaged. For this, special rollers are used that have segments with different diameters so that additional transverse stresses are applied to the paper.
According to JP 02122967 A, folds or waves in the paper are measured by means of an ultrasonic sensor. The height of the print heads is adjusted depending on the determined height of the paper. Here it is also avoided that the print heads come into contact with the paper, whereby they may be damaged.